This invention relates to an induction motor, or more in particular to a stator structure of the induction motor.
One of the conventional methods for reducing the cost of a small induction motor is by reducing the overall size of the coil form and lessening the peripheral winding length of the coil inserted into the stator iron core, in order to save the amount of magnet wire. In view of the fact that the peripheral length of the winding inserted into the iron core of the stator is minimized as far as required, the forward end of the coil does not expand radially outward but extends substantially linearly. In other words, each conductor element of the winding passes along the shortest path with a U-shaped section. This is especially the case when the number of poles involved is small as in the case of a two-pole motor. For this reason, it is impossible to bind the coils with a braided rope by a lacing machine as is generally employed to fix the coil end. To overcome this problem, a coil-supporting member of plastics has been suggested as disclosed in the West German patent application Laid-Open No. 2,018,125. According to this prior art, the high moldability and elasticity of plastic material are utilized. In other words, a coil-supporting member of plastics molded into the shape along the inner surface of the short-passed coil end portion and also into the shape securing a sufficient distance from the rotor is inserted through the rotor-accommodating cavity formed in the stator iron core from the opening of the bell-shaped coil end portion and fitted on the inner surface of the short passed coil end portion. The coil-supporting member thus fitted supports the short-passed coil end portion directly, thereby preventing the winding from being loosened in the process of manufacture or in operation on the one hand while at the same time defining the opening for securing the passage of the rotor shaft on the other hand.
As a result of an aging test conducted on the motor as configured above, it has been found that with passage of time, contraction of the coil-supporting member and the out-of-phase condition of relative vibrations of the short-passed coil end portion and the coil-supporting member causes a backlash or looseness therebetween. This backlash is a source of not only a noise but a friction between the coil enameled wire and the coil-supporting member to damage the insulating cover of the enameled wire of the coil. In an extreme case, the coil-supporting member is brought into contact with the rotor or rotor shaft and broken, with the result that the broken pieces of the supporting-member intrude into the air gap between the rotor and the stator iron core, often locking the motor.
In order to solve these problems, the inventors discussed the use of a thermo-setting resin in place of the thermo-plastic as the material of the coil-supporting member in vain. Since the thermo-setting resin is less elastic than the thermo-plastic, it is not easy to insert the coil-supporting member of the thermo-setting resin from the bell-shaped coil end side. Further, it cannot be successfully fitted on the inner surface of the short-passed coil end portion.